An ethylene-vinyl alcohol copolymer, which may hereinafter be referred to as an EVOH, has a considerably smaller oxygen transmission rate than other plastics and is of good melt shapability. Therefore, it is widely used as food packaging materials. It has recently been used for applications other than food containers such as fuel tanks of automobiles and containers of agrochemicals, because of its chemical resistance and small permeability of various chemicals. However, there is a problem that when a packaging material containing an EVOH is used for applications where retort treatment under high temperature and high humidity is performed, whitening, deformation or deterioration in barrier property occurs.
In order to improve such hot water resistance and the like, various methods have been proposed with regard to a technique of crosslinking an EVOH. For example, Patent document 1 discloses that a compound having an epoxy group and an allyl group is incorporated into an EVOH and then crosslinking is performed by application of light or heat. However, judging from the hot water breakage temperature in Examples of Patent document 1, only a small effect is obtained and almost no crosslinkage seems to be formed. It is conceivable that this is caused by the fact that almost no epoxy group has reacted with the EVOH. Moreover, in producing the compound, it is necessary to add a large amount of a compound having an epoxy group and an allyl group. Therefore, it is feared that this compound remains and it will cause health problems particularly when the material is used for food packaging containers.
Patent document 2 and Patent document 3 describe adding at least one crosslinking agent and crosslinking aid selected from among multifunctional allyl compounds, multifunctional (meth)acrylic compounds, polyhydric alcohols and metal oxides to an EVOH and irradiating it with an electron beam to crosslink. However, this is also feared to have health problems caused by remaining of additives. Moreover, the crosslinking agent reacts with an EVOH in the course of melt-kneading to cause gelation. Therefore, there is a problem with prolonged operation.
Patent document 4 describes adding a compound having two or more allyl ether groups to an EVOH and irradiating it with an electron beam to crosslink. However, this also appears to have health problems caused by remaining of additives.
Patent document 5 describes a method in which triallyl cyanurate and triallyl isocyanurate are used as crosslinking agents and these are melt-kneaded with an EVOH and then the EVOH is crosslinked by irradiation with an electron beam. However, the triallyl cyanurate and the triallyl isocyanurate will remain. Therefore, there is a fear of health problems particularly in use for food packaging containers. Moreover, the triallyl cyanurate and the triallyl isocyanurate react with an EVOH in the course of melt-kneading to cause gelation. Therefore, there is a problem with prolonged operation.
Patent document 6 describes a method of crosslinking an EVOH film by bringing it into contact with water to make it containing water and then irradiating it with an electron beam. However, in this method, it is necessary to immerse a film in water for a long time, and therefore there is a problem that it is difficult to perform high-speed production.
Patent document 7 describes to cause a specific epoxy compound to react with an EVOH to modify it, thereby improving its flexibility while maintaining its gas barrier property as much as possible. However, there is a drawback that the modification leads to a significant drop of melting point, and therefore it is difficult to use that as it is in applications where heat resistance is required. Patent document 8 describes a resin composition comprising a modified EVOH described in Patent document 7 and an unmodified EVOH.
Patent document 1: JP 63-8448 A
Patent document 2: JP 5-271498 A
Patent document 3: JP 9-157421 A
Patent document 4: JP 9-234833 A
Patent document 5: JP 62-252409 A
Patent document 6: JP 56-49734 A
Patent document 7: WO 02/092643
Patent document 8: WO 03/072653